1) Field of the Invention
The present invention relates to an image forming apparatus (such as copy machine, printer, and facsimile) that has a minimum number of condensers (such as capacitor) to prevent temperature drop of a fixing device (such as fixing roller) due to continuous paper delivery.
2) Description of the Related Art
An image forming apparatus, such as copy machine that forms an image on a recording medium such as ordinary paper and over head projector (OHP), frequently employs the electrophotographic system in view of high-speed, image quality, cost, and the like. The electrophotographic system is a method in which a toner image is formed on a recording medium, and the formed toner image is fixed on the recording medium by heat and pressure. As to a fixing system, a heat roller system is employed most frequently at present in view of safety and the like. In the heat roller system, a mutually pressed portion referred to as a nip portion is formed by pressing a heat roller heated by a heating member such as halogen heater against a pressure roller arranged opposite to the heat roller, and a recording medium on which a toner image has been transferred is heated and pressed while passing through this nip portion, thereby fixing the toner onto the recording medium.
In recent years, an environmental issue has become important, which leads to energy saving in the image forming apparatus such as copy machine and printer. When energy saving of these image forming apparatuses is taken into consideration, what cannot be ignored is power saving of a fixing device that fixes toner to a recording medium. More power saving is demanded by reducing energy consumption during standby (when the apparatus is not in use), in concrete terms, reducing to zero.
However, when the energy consumption is made zero during standby, a long heating time, for example, a few minutes to over ten minutes is needed to raise the temperature to a usable temperature of approximately 180 degrees C. because a metal roller made of, for example, iron or aluminum is mainly used for the heat roller of the fixing device, and therefore the thermal capacity is large. Such a waiting time worsens the usability of the apparatus for a user. Accordingly, a heating system in which power consumption is as small as possible, while startup is fast from a standby state has been desired.
To shorten the time taken to raise the temperature of the heat roller, it is obvious that making supplied energy per hour, that is, rated power large is good. In practice, many image forming apparatus called a high-speed machine of which printing speed is fast are operated with 200 volts for their power voltage. However, commercial power used in common offices in Japan is 100 volts and 15 amperes; therefore, special work is required for power-related facility at an installation site of the machine in order to deal with 200 volts. Adaptation to 200 volts is not regarded as a common resolution.
As long as commercial power of 100 volts and 15 amperes is used, maximum supplied energy is determined due to the power source even though the temperature of the heat roller is tried to be raised within a short time. Thus, in a conventional technology disclosed in Japanese Patent Application Laid-Open Publication No. H10-10913, temperature of a heat roller is constantly maintained slightly lower than a fixing temperature during standby, and the temperature is immediately raised to a usable temperature when the image forming apparatus is used. Therefore, a user does not have to be kept waiting until the temperature of the fixing roller is raised. This technology is commonly used for reduction of power consumption of the fixing device during standby of the image forming apparatus.
However, a certain amount of power must be supplied to the fixing device even when the fixing device is not in use. It is said that energy consumption during standby corresponds to about 70 to 80 percent of the whole energy consumption of devices constituting the image forming apparatus. Thus, extra energy is disadvantageously consumed, and energy saving is not sufficient. Further, the technology does not aim at increasing the maximum supplied power at the startup more than the power supplied from the main power source.
On the other hand, in a conventional technology disclosed in Japanese Patent Application Laid-Open Publication No. H10-282821, a secondary battery serving as an auxiliary power source is charged during standby of the fixing device, and power is supplied from a main power source and the secondary battery or a primary battery when the fixing device is started up, thereby the rise time is shortened. As the secondary battery, a lead-acid battery, a nickel-cadmium battery, or a nickel-hydrogen battery is generally used. The capacity of such a secondary battery is degraded and reduced when charge and discharge are repeated (i.e. memory effect). The life of such a secondary battery is shortened as power is discharged in a larger current. Even for batteries that are said to have generally a long life, when discharge is carried out in a large current, the number of repetition of charge and discharge is approximately 500 to 1,000. If charge and discharge is repeated 20 times a day, the life of the battery runs out in about one month. Therefore, battery exchange is carried out more frequently, which makes a trouble and increases a running cost such as cost of batteries to be exchanged. Furthermore, lead-acid batteries use liquid sulfuric acid for electrolyte and so forth, and therefore, there is also an undesirable aspect that they are not suitable for office machines.
Still further, there are problems that not only is the load to the heating circuit built in the heat roller increased by an abrupt current change, inrush power, and the like but also noise is generated owing to flow of the inrush current to peripheral circuits when supply of large power is started and stopped. Accordingly, frequent turning on and off of power supply from an auxiliary power source with a large capacity is not desirable. Moreover, when a large amount of power is supplied at one time, the supply becomes excessive, which may lead to a possibility that the temperature of the heating circuit rises too high.
A device that uses a capacitor capable of charging and discharging an auxiliary power source, has a charger that charges the capacitor of the auxiliary power source with power supplied from a main power source, and a switching unit that switches between the charge of the auxiliary power source and the power supply from the auxiliary power source to a secondary heating element and adjusts electric energy supplied from the auxiliary power source to the secondary heating element has been proposed as a fixing device that can improve the above problems, enhance a power saving effect, reduce the noise caused by an inrush current and abrupt current change at the time of large power supply, shorten the rise time, and prevent the temperature from rising too high (see, for example, Japanese Patent Application Laid-Open Publication No. 2002-184554).
In other words, the objects to supply power of the capacitor are as follows: First, supply of the capacitor power at the time of startup of a copy machine and the like allows supplying power exceeding the amount that can be supplied from commercial power to the fixing device, thereby making it possible to shorten the rise time; Next, although there has been a problem that a thin fixing roller with a small thermal capacity cannot be used in a high-speed machine because a large amount of heat of the fixing roller is lost from the recording member, which makes the temperature of the fixing roller appreciably low, supply of the capacitor power at the time of temperature drop of the fixing device allows the temperature drop to be prevented and a thin roller to be used in a high-speed machine.
On adoption of a condenser such as capacitor described above, prevention of fixing failure is the highest priority. Therefore, in accordance with specifications of copy machines, the electrostatic capacity of the capacitor is set so that the temperature drop of the fixing roller does not occur how large the number of sheet of continuous paper feeding is and whatever paper is. Further, the price of a capacitor is very high at present, and therefore, mounting of capacitors more than required gives rise to unnecessarily high cost of an image forming apparatus such as copy machine. Thus, the number of capacitor to be mounted is preferred to be minimally necessary. However, due to the above problems, determination of an optimum electrostatic capacity is difficult, resulting in that capacitors have been mounted more than required in conventional machines. Therefore, condensers such as capacitor have not been used efficiently so far.